1. Field of the Invention
The present invention is in the field of tin plating. More particularly, this invention is in the field of surface-active additives for high-speed continuous tin and tin-alloy plating based on methanesulfonic acid.
2. Description of the Prior Art
High-speed electroplating equipment and processes for depositing tin and tin alloys are well-known in industry, and generally consist of processing the work to be plated through an appropriate electroplating solution in an electroplating cell. The electroplating solution is cycled from a reservoir into the electroplating cell to provide vigorous agitation, solution circulation and chemical replenishment.
The electroplating solution should possess a number of features for effective operation in this type of processing, these include: the ability to electroplate the desired deposit at high speed; production of a lustrous and fine-grained deposit, even at the high current densities which are required for high-speed plating; ability of the deposit to execute uniform fusing or melting and thereby demonstrate good solderability of the deposit; stability of the solution and its components to low pH and air which is generally introduced due to the vigorous solution movement in high-speed plating; and solution clarity, i.e., freedom from turbidity, even at temperatures above 50 degrees Centigrade (.degree.C.).
Due to the high current densities involved and relatively low solution volumes, the temperature of the bath in high-speed electroplating methods increases until the solution reaches equilibrium at an elevated temperature with its ambient environment. Additives must not cause solution turbidity at elevated temperatures above their cloud point.
Because of vigorous solution movement and mixing with air, there is a strong tendency to produce a foam, which can be detrimental to the electroplating process. The additives used should generate as little foam as possible in the plating equipment, and preferably should generate none at all.
Many electrolytes have been proposed for electroplating tin and tin alloys; one of these is described in U.S. Pat. No. 4,701,244, to Nobel et al. This patent discloses the electroplating of tin, lead or tin/lead alloys from lower alkyl sulfonic acid baths which contain brightening additives and wetting agents of various types. Surfactants disclosed in that patent comprise betaines, alkylene oxide polymers, imidazolinium compounds, quaternary ammonium compounds, ethylene oxide derivatives of amines, phosphonates and amides.
U.S. Pat. No. 4,673,470 describes a tin, lead, or tin/lead alloy plating bath based upon an aliphatic or aromatic sulfocarboxylic acid. Instead of the alkane or alkanol sulfonic acids disclosed in previous patents, this patent includes a carboxylic acid radical in the organic sulfonic acid compound. The electroplating baths described contain brightening agents plus a surface-active agent, also known as a surfactant, with particular emphasis on those agents which are non-ionic. A very broad group of non-ionic surface-active agents is described as being useful, and a wide range of such wetting agents is listed.
In all of the prior-art baths which have been proposed, the wetting agents described as being useful for producing either bright or matte deposits are very broadly described, and are deemed somewhat equivalent to one another. Numerous examples are given in each of the referenced patents, directed to a wide variety of agents of many different types, most of which contain some type of ether or similar condensation compound.
Most of the prior-art surface-active agents are unsuitable for high-speed plating in modern high-speed plating equipment. These agents are generally incapable of satisfying all of the requirements for the electrolytes listed above.
Tin plating is a well-known method of protecting steel from corrosive attack in containers for packaging food, especially those of a relatively corrosive medium such as, e.g., tomato products, processed pineapple, cherries, and the like. Because of the very high volume of products packaged in tin-plated steel cans, the amount of tinned steel is correspondingly large, and the tinning process, to be economical and effective, must be rapid and thorough. While processes to deposit tin on a steel surface are known, there are a number of problems to which attention must be given. The tin deposit must cover the steel surface thoroughly, and with a minimal porosity through which attack on the steel surface can occur. The problem of corrosive attack on the steel can be partially met by increasing the thickness of the coverage, but this approach is too costly.
Those skilled in the art are aware that there are a number of properties which an electroplating solution for steel should possess to permit reliable, economical high-speed plating. As noted, these properties include clarity, or freedom from turbidity; stability to air and strong acid; a minimal or zero tendency to produce foam; and the ability to provide a lustrous, fine-grained deposit, even under plating conditions involving high current densities. Further, the tin coating in its final state on the steel should have a good ability to be remelted and soldered.
In my U.S. Pat. No. 4,662,999, I describe a bath for the electrodeposition of tin and tin-containing alloys. That bath is free of fluoride and fluoborate ion, and contains alkylsulfonic acids and non-ionic, cationic, anionic and amphoteric surfactants and brightening agents. Other United States patents in this field include Nobel et al., U.S. Pat. No. 4,717,460; Toben et al., U.S. Pat. No. 4,880,507; and Kroll et al., U.S. Pat. No. 4,923,576.
Johnson, in U.S. Pat. No. 3,860,502, discusses ethoxylated naphthol as a surfactant in the high-speed tin plating of strip steel. He notes that while relatively short molecules provide little foaming but somewhat poor solubility, improving solubility by simply increasing the degree of ethoxylation, i.e., the amount of ethylene oxide added per molecule of naphthol, leads to excessive foaming and a requirement for very high current densities. He therefore sulfonates the molecule and limits the degree of ethoxylation to the range of about five to seven.